Step-by-step explanation:
Similarity: In both cases, a simple machine-like mechanism is seen which is similar to a mechanical lever.
Difference: In the first image, the forces acting on the feet of the men are balanced, and hence both the men are horizontally balanced indicating the balancing of force on both the ends. In the second image, the piston of the crane acts as a fulcrum. The force due to the weight of the metal bar hanging is not enough or is less than the force on the bottom of the crane. The forces are not balanced.
In the first case, if the upper man moves to the right and holds around the knees of the second person, the balance would collapse because the force acting on the feet of the second person will then be higher. Similarly in the second case, if the hanging weight of the metal bar is increased so that it is greater than the weight of the crane bottom. The crane will collapse.
The main reason behind the collapse is the unbalanced force on both ends of a lever.